To overcome this challenge, another category of grant-free random access, i.e., unsourced random access (URA), was proposed in [1]}.
Indeed, the roll-out of URA is motivated by practical Internet-of-Things scenarios, where millions of low-cost devices have a common codebook at the moment of production [1]}. Active devices maps their messages to codewords of the common codebook, and the BS recovers the messages by detecting the transmitted codewords, without determining the identities of active devices. If active devices wish to reveal themselves, they can embed identity information in their payloads. In order to decrease the size of codebook, active devices usually partition their messages to several sub-messages [3]}, [4]}. Active devices adds some parity check bits to each sub-message which is mapped to a codeword of the common codebook, and then the BS recovers all the sub-messages which are stitched to the original messages by using a some decoder according to the parity check bits, e.g., the tree-based decoder [3]}. Compared to sourced random access, URA has a higher spectral efficiency, since it does send pilot sequence for identity detection [6]}. Moreover, URA has a lower computational complexity, which is independent of the total number of potential devices and depends only on the cardinality of the active device set [6]}. Hence, URA is appealing to 6G wireless networks with a large number of devices with sporadic activities.
